An analog-to-digital converter (ADC) converts a continuous voltage signal into a time-varying sequence of digital numbers. This permits digital devices, such as computers, to process real world signals, such as measurements of sound and temperature. Many different types of ADCs are known, including sigma-delta converters (SDC).
FIG. 1 presents an example of a prior art first-order SDC. In this SDC, a continuous voltage signal is oversampled and provided to a adder 100 that is configured to receive the output of the converter as a feedback signal. The adder 100 subtracts the converter output from the oversampled input and provides the difference to an integrator 104. The output of the integrator 104 is provided to a quantizer 108.
The output of the quantizer 108 is fed back to the adder 100 through a digital-analog converter 112, facilitating the operation of the adder 100. The output of the quantizer 108 is typically provided to a digital filter (not shown) for decimation before it is used in subsequent processing.
One drawback to SDC technology is that the RMS noise characteristics of a single SDC stage are proportionate to the reciprocal square root of the oversampling ratio, which is itself a function of the sampling frequency applied to the continuous voltage signal. The requirement of a high oversampling ratio to reduce noise makes an SDC better suited to low frequency applications than high frequency applications.
Accordingly, there is a need for methods and apparatus that provide improved ADC techniques.